This invention relates to a speed detector, and more particularly to a speed detector which is highly suited for use in detecting the rotational speed of an induction motor.
Induction motors which are capable of being controlled in speed are now being employed to drive machine tools and other types of equipment. The speed control mechanism for an induction motor includes a variable voltage and/or variable frequency drive section which relies upon thyristors or transistors, a speed command section which issues commands that establish the desired rotational speed, a speed control circuit, a speed detector for detecting the rotational speed of the induction motor, and a speed feedback circuit which supplies the speed control circuit with a feedback signal indicative of the actual speed of the induction motor as detected by the speed detector. The speed control mechanism makes use of such an arrangement to effect automatic control of the induction motor so as to make it follow the command speed at all times.
While induction motors require little maintenance because of their simple and sturdy structure, this is not the case with the conventional speed detectors that are used to detect the speed of such motors. Specifically, the conventional speed detectors employ excitation DC-type or permanent magnet DC-type configurations having brushes and require that a considerable amount of time and labor be devoted to maintenance. Such speed detectors, when applied to the relatively maintenance-free induction motor, tend to cancel the merits of the induction motor.